1. Field of the Invention
The present invention relates to a thermal gas sensor configured to measure components of a gas to be measured, based on a variation in heat conduction in the gas.
2. Background Art
A thermal gas sensor is used to analyze a gas using a variation in heat conduction in the gas. The variation in the heat conduction in the gas is measured based on the amount of heat radiated by a heating member exposed to the gas.
Thermal gas sensors are used in various technical fields. For internal combustion engines for automobiles and the like, there has been a demand to accurately measure the flow rate, temperature, and pressure of intake air as well as an environmental condition such as humidity in order to reduce fuel consumption. Thermal gas sensors are also used to detect the concentration of hydrogen in internal combustion engines for automobiles that use hydrogen as fuel in order to allow the internal combustion engine to be optimally driven.
Thermal gas sensors serving as gas sensors that measure the humidity or the concentration of gas as described above avoid absorbing moisture and are excellent in environment resistance such as contamination resistance and prolonged stability. JP Patent No. 2889909 as a prior art document for such a thermal gas sensor discloses a humidity sensor based on a variation in the resistance value of a resistor heated in an atmosphere and configured to compare a voltage generated across the resistor at a high temperature with a voltage generated across the resistor at a low temperature to sense the humidity, wherein at the low temperature, the variation in resistance is affected only by the atmosphere temperature, and at the high temperature, the variation in resistance is sensitive to the temperature and humidity of the atmosphere.
Furthermore, JP Patent No. 3343801 discloses a humidity sensor including heating means for heating a temperature-sensitive resistor using a heating member. The heating means applies two pulse voltages to the heating member in order within a given time to switch the temperature of the temperature-sensitive resistor between a first value of at least 300° C. and a second value of 100° C. to 150° C. The humidity sensor detects the humidity based on an output voltage associated with a drop in the voltage of the temperature-sensitive resistor at each of the temperatures.
The humidity sensors disclosed in JP Patent Nos. 2889909 and 3343801 are configured to heat the same heating member or temperature-sensitive resistor to the low temperature (second temperature) and to the high temperature (first temperature) in a time division manner.
The configuration that heats the same heating member or temperature-sensitive resistor in a time division manner as described above has the advantage of saving power. However, this configuration is disadvantageous in that an amount of time is required to heat and naturally cool the heating member or temperature-sensitive resistor to the different temperatures, thus reducing response speed.
In particular, the measurement of humidity of intake air for an internal combustion engine requires instantaneousness because the humidity is important data used for the instantaneous calculations of fuel injection time and the like. In connection with such an application, the response speed is a challenge for the conventional humidity sensors disclosed in JP Patent Nos. 2889909 and 3343801.
Furthermore, a signal corresponding to the humidity is conventionally obtained by calculations that use various parameters such as the output voltages associated with drops in the voltage of the temperature-sensitive resistor at the first and second temperatures and the premeasured resistance value of the temperature-sensitive resistor. This is to eliminate the adverse effect of a variation in gas temperature. Thus, when prolonged use deteriorates the resistance value of the temperature-sensitive resistor, errors disadvantageously occur in the parameters to increase the errors in calculations.
Thus, an object of the present invention is to solve the problems with the above-described conventional examples and provide a high-responsiveness and high-accuracy thermal gas sensor that can be used in various environmental conditions.